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Roger Federer has a touch of Roman emperor about him. It’s that Roman nose, the statuesque torso, the fixed regal gaze, the headband doubling as a laurel wreath, but also the thought he gives to his place in history.

“I am the best player of this generation, but as for my position in the history of the game, I am not even near the best,” he mused after pocketing his latest US Open. The scroll of victories on his triumphal arch is still too short.

But victory in the French Open that begins in just over a week’s time could be enough to crown him as the greatest ever. The clay surface of Roland Garros is the only grand-slam tournament he hasn’t yet conquered – or as the French sports newspaper L’Equipe says: “One small village of indomitable Gauls still resists him.”

Federer admits: “Roland Garros has become the great objective.” There is only one hitch. In the final in Paris on June 11 he will probably confront arguably the best clay-court player ever, the Spaniard Rafael Nadal. This will be Federer’s Rubicon.

The roll-call of men who have won all four grand-slam tournaments is a dusty one: Fred Perry, Donald Budge, Rod Laver, Roy Emerson and Andre Agassi. All other great players had one tournament they couldn’t conquer. For Bjorn Borg it was the US Open, for Ivan Lendl Wimbledon, and for John McEnroe, Pete Sampras and Federer, it’s Paris.

The tournament is Federer’s equivalent of the slave who tells the emperor: “Remember you are mortal.” And yet he is equipped for a Gallic campaign. Unlike Sampras and McEnroe, Federer grew up on clay. Unlike Borg in New York, he feels at home in Paris. French is one of the three languages in which he exhorts himself on court, and Paris is practically his native slam, always drawing an entourage from his home town of Basle. Yet he has never reached a final there.

One problem is that he is too gifted for clay. He is used to finishing points in a couple of shots, but clay is so slow that the opponent can return more balls. Suddenly Federer needs to rally. Spaniard Sergi Bruguera, winner of two French Opens, said the same problem undid Sampras: “He was winning easily without suffering, and then on clay you can’t. You have to suffer. It’s difficult to change your mentality.”

But Federer is trying. When I asked what he would do differently in Paris this year, he said: “I’ll try to be mentally prepared for long and tough matches and rallies. I think in my game, that’s the key to success.”

His other problem is Nadal. Federer has won 39 matches this year and lost three. All three were finals, all to Nadal. When the 19-year-old beat him in Rome on Sunday, it was his 53rd consecutive victory on clay, equalling Guillermo Vilas’s record from 1977.

Being left-handed, Nadal naturally hits his forehand high and bouncing to Federer’s backhand – the nastiest shot in tennis to deal with. Worse, the pretender is acquiring a psychological grip over the emperor. Nadal exudes energy. Even hours into a match he bounds around between points. This is a new proposition for Federer, who is used to quelling others with a glance. In tennis the mental game is crucial: there is only one opponent, you face him for hours, and the stops and starts of play give you ample time to think yourself into defeat.

Federer denies this is happening to him against the man who is essentially his only opponent: “Rafael doesn’t have such a great psychological advantage over me. I am 2,000 points ahead of him in the rankings, and that counts for as much as my three straight defeats to him.”

In any case, Federer can overcome his past because he is such a good tennis thinker. The conventional wisdom in sport is that during a match you must be “in the zone”, playing without reflecting. Federer doesn’t believe this. Part of him always sits in the stands watching himself play, working out what to change. “I always adapt to my opponents and I usually adapt to the surface,” he told me. Recently on clay he has begun coming to the net more.

If he needs motivation in Paris, he should pop into the English-language bookshop on Rue de Rivoli and buy McEnroe’s autobiography, Serious. In it, McEnroe recalls leading Lendl by two sets to love in the 1984 final of Roland Garros and then losing.

“It’s even tough for me now to do the commentary at the French,” McEnroe writes. “I’ll often have one or two days when I literally feel sick to my stomach at just being there and thinking about that match.” Winning Paris “would have given me that final, complete thing that I don’t have now – a legitimate claim as possibly the greatest player of all time.”

You can imagine Federer reading this in the back of the courtesy car and trembling, while a slave holds the laurel wreath over his head.